Interpretive Summary: One of the key issues in controlling the navel orangeworm is determining its rate of development in the field, which is complicated because this insect is a pest of both almonds and pistachios and may develop at different rates on each nut species. Previous research concentrated on a single variety of almond and it was assumed that the duration of development was the same on all almond varieties and pistachios. A series of laboratory and field experiments were conducted to determine if there were differences in development among almond varieties as well as between almonds and pistachios. Additional studies were conducted to determine if development was faster at constant temperature than at fluctuating temperatures. In these studies development was faster and survival greater at constant temperature, and the differences were greatest when the fluctuating temperatures approached 111°F. When conditions were equal, development was faster on pistachios than on almonds, but on all nuts some individuals required two to three times longer to develop than the first adults that emerged. This variation was greatest in almonds and overall, the speed of development depended on the quality of the nuts. Development was fastest on new crop nuts and took longer on unharvested nuts from the previous year. Among almonds, navel orangeworm developed at different rates on different varieties, and its survival also depended on variety. We consider the almond varieties that had the slowest rate of navel orangeworm development and survival to be resistant and future experiments will attempt to quantify the factors responsible.

Technical Abstract:
A series of laboratory and field studies were conducted using three lines of navel orangeworm (NOW) reared on wheat bran diet, pistachios, and almonds at constant and fluctuating temperature. The duration of development, measured in degree-days Celsius (DD°C) differed significantly among the three lines on wheat bran diet at constant temperature. Development was as much as 40% faster at constant temperature than at fluctuating temperatures in the laboratory and in the field, therefore the developmental duration determined at constant temperature was not an absolute measure even when the same number of DD were accumulated at constant and varying temperature. Survival decreased by 50% compared to the constant temperature control when the maximum temperature in fluctuating regimes exceeded 43°C. In pistachios, NOW developed faster on new-crop nuts than on unharvested nuts (mummies) and survival was the greatest, 57%, on mature nuts, and the lowest, 3.5%, on mummies. In almonds, the development rate on new-crop nuts was variety-dependent and was the slowest on the experimental selection ‘23-122’. Development and survival on almond mummies was also variety-dependent, and NOW emerged earliest from ‘Nonpareil’ and latest from ‘Carmel’ nuts, differing by 529 DD°C. Survival on almond mummies was the highest on ‘Butte’, 35.7%, and the lowest on ‘Carmel’, 7.2%. When pistachios and almonds were compared, development on new-crop pistachios was 25%-100% faster than on almonds, depending on variety, and the duration of development of NOW on both pistachio and almond mummies were similar. In our trials, both the speed of NOW development and survival depended on nut age, variety and quality, indicating that nuts are a dynamic rather than a static nutrient source for NOW.